METHOD AND APPARATUS FOR INTERCONNECTING SOLAR CELLS

Abstract: A method and apparatus are provided for interconnecting solar cells by means of electrically-conductive ribbons. The method and apparatus are characterized by use of (a) means for feeding cells from a cell loading station to a cell soldering station via a cell receiving station; (b) means for feeding one or more electrically-conductive ribbons from one or more supply rolls to a cell in said soldering station, so that said one or more ribbons overlie a cell in said soldering station and can underlie a subsequently received cell in said receiving station; and (c) separate means for severing and soldering said ribbons whereby discrete sections of said one or more ribbons are formed, with each discrete section having a leading end soldered to one contact of a first cell and a trailing end soldered to the second contact of a second cell. An optional feature is to form an offset crimp in the ribbons between cells so as to provide strain relief.

Description: METHOD AND APPARATUS FOR INTERCONNECTING SOLAR CELLS This invention relates to the art of inter¬ connecting photovoltaic solar cells and more particularly to a novel method and apparatus for interconnecting solar cells by means of flexible electrical conductor ribbons. BACKGROUND OF THE INVENTION The manufacture of photovoltaic solar cells involves provision of semiconductor substrates in the form of sheets or wafers having a shallow p-n junction adjacent one surface thereof (commonly called the "front surface") . Such substrates, which may include an insulating anti-reflection ("AR") coating on their front surfaces that is transparent to solar radiation, are commonly referred to as "solar cell blanks" or "solar cell substrates". In the case of silicon solar cells, the AR coating is usually made of silicon nitride or an oxide of silicon or titanium. By way of example but not limitation, a typical solar cell blank may take the form of a rectangular EFG-grown polycrystalline silicon substrate of p-type conductivity having a thickness in the range of 0.012 to 0.018 inches and a p-n junction located about 0.5 microns from its front surface, with a silicon nitride coating about 800 Angstroms thick covering its front surface. Other substantially equivalent solar cell blanks also are well known, e.g. those comprising single crystal silicon substrates and cast polycrystalline silicon substrates. The solar cell blanks are converted to finished solar cells by providing them with electrical contacts (sometimes referred to as "electrodes") on both the front and rear sides of the semiconductor substrate, so as to permit recovery of an electrical current from the cells when they are exposed to solar radiation. These contacts are typically made of aluminum, silver, nickel or some other electrically conductive metal or metal alloy. A common arrangement is to provide silicon solar cells with rear contacts made of aluminum and front contacts made of silver. The contact on the front surface of the cell is generally in the form of a grid, comprising an array of narrow fingers and at least one elongate bus (also sometimes called a "bus bar") that intersects the fingers. The width and number of the fingers and busses are selected so that the area of the front surface exposed to solar radiation is maximized. Often the front grid contact has two parallel bus bars. The AR coating overlies and is bonded to those areas of the front surface of the cell that are not covered by the front contact. Depending on how the front contact and the AR coating are formed, the AR coating may also cover a substantial portion of the front contact. The rear contact may cover the entire rear surface of the solar cell blank, but often it is formed so as to terminate close to but short of the edges of the blank. Aluminum is preferred for the rear contact for cost and other reasons. However, to facilitate soldering a connecting wire lead to the contact, it has been found useful to provide apertures in the aluminum coating, with silver soldering pads filling those apertures so as to slightly overlap the adjacent aluminum layer. The silver pads form oh ic bonds with the underlying substrate and also low resistance electrical connections with the aluminum contact, and are used as sites for making soldered connections to the rear contact. The silver soldering pads are considered to be an integral part of the rear contact. Such a contact arrangement is disclosed in PCT International Publication No. WO 92/02952, based on U.S. Patent Application Serial No. 07/561,101, filed September 1, 1990 by Frank Bottari et al for "Method Of Applying Metallized Contacts To A Solar Cell". An alternative but similar back contact arrangement wherein the aluminum coating has apertures filled with silver soldering pads involves having the aluminum overlap the edges of the silver soldering pads. This arrangement is achieved by forming the silver soldering pads before the aluminum coating is formed. It also is possible to bond wire leads of copper or a copper alloy to the aluminum contact without need for silver soldering pads as described. However, in such event the aluminum contact is usually coated with one or more metal layers, including a metal layer that is wet by solder, e.g., layers of nickel, copper and tin as described in U.S. Patent No. 4451969 issued 5 June 1984 to A.R. Chaudhuri for "Method Of Fabricating Solar Cells". How the grid-shaped contact and the AR coating on the front surface are formed is not critical to this 387 PCI7--B94/00446 _ 4 _ invention. They may be formed in various ways, as exemplified by U.S. Patents Nos. 4451969, 4609565, 4751191, 5010040, 5074920, British Patent No. 2215129, and PCT International Application WO 89/12321, published 14 December 1989. Regardless of how the front grid contact and the AR coating are formed, at least a portion of the front contact is not covered with the AR coating, so as to permit making a soldered connection to that contact. For the purposes of this invention it is preferred that at least selected portions of the bus bars of the front contact not be covered by any material except for solder in order to facilitate cell interconnections. Photovoltaic solar cells (e.g., silicon solar cells) are typically small in size, e.g. , 2-4 inches on a side, with the result that their power output also is small. Hence, industry practice is to interconnect a plurality of cells so as to form a physically integrated module with a correspondingly greater power output, and then several such modules are in turn assembled and interconnected to form a multi-module array, sometimes called a "solar panel". Several multi-module arrays or panels may be connected together to form a larger array or panel. For various reasons including convenience of manufacture and assembly, cost control, and physical protection of the individual cells and their interconnections so as to extend the useful life of the cells, it has been common practice to provide modules in the form of laminated structures. These laminated modules consist of front and back protective sheets, with at least the front sheet being made of clear glass or a suitable plastic material that is transparent to solar radiation, and the back sheet being made of the same or a different material as the front sheet. Two or more strings of solar cells are disposed between the front and back sheets, together with a transparent polymer material that encapsulates the solar cells and is also bonded to the front and back sheets, so as to form a laminated sandwich-style module. Such laminated sandwich-style modules mechanically support the brittle silicon cells and also to protect the cells against environmental degradation. U.S. Patents Nos. 4239555, 4692557, and 5110369 disclose various solar module constructions. A known practice is to use electrically conductive wire leads to electrically interconnect a plurality of cells in a string, with the cells in the string being disposed in a row and connected electrically in series, and then to interconnect two or more strings to form a module, the several strings in the module being connected in parallel or in series, or with some strings being connected in parallel and others in series, and two or more modules in an array may be connected in series or in parallel, depending on the voltage and current output that is desired from each module or the multi-module array. A known practice is to use copper wire, preferably in the form of strips of flat pre-tinned copper ribbon, as the wire leads for interconnecting a plurality of cells in a string, with each ribbon being soldered to the front or back contact of a particular cell by means of a suitable solder paste, e.g., a solder paste as described in U.S. Patent No. 5074920. The solder paste is deposited onto the contacts at ambient temperatures, preferably as discrete small daubs. Thereafter each cell is connected to another cell, or its contacts are provided with electrically conductive connecting leads, by positioning a pre-tinned copper ribbon along a cell contact in engagement with the solder daubs on that contact, and then heating the ribbon, cell and solder daubs just enough, preferably by exposure to hot air, to drive off the fluxing agent and cause the metal components of the solder daubs to fuse the copper ribbon to the adjacent solar cell contact. OBJECTS AND SUMMARY OF THE INVENTION The primary object of this invention is to provide a new and improved method and apparatus for interconnecting solar cells by means of flexible electrically conductive wire. Another object is to provide a novel method and apparatus for interconnecting solar cells that have electrical contacts pre-coated with a solder material. A further object is to provide a novel method and apparatus for efficiently and reliably interconnecting solar cells into strings by means of flexible electrical conductors in the form of wires or ribbons without subjecting the latter to excessive stresses, whereby to avoid premature rupture of the conductors or their connections to the interconnected cells. A more specific object is to provide a method and apparatus for interconnecting adjacent solar cells by eans of flexible electrical conductors characterized by means for crimping the electrical conductors between the interconnected cells. Another object is to provide a method and apparatus for soldering flexible electrical conductors to the front and rear contacts of a solar cell simultaneously. Another object is to provide a method of interconnecting photovoltaic cells with flexible electrical conductors whereby the conductors are soldered to the solar cells using jets of heated air followed by cooling air. A further object is to provide a method and apparatus for interconnecting solar cells having front and back contacts characterized by use of (a) means for feeding cells to a cell soldering station via a cell receiving station, (b) means for feeding one or more electrical conductors from one or more supply rolls to a cell in said soldering station, so that said one or more electrical conductors overlie a cell in said soldering station and can underlie a subsequently received cell in said receiving station, and (c) separate means for severing and soldering said conductors whereby discrete sections of said one or more electrical conductors are formed with each discrete section having a leading end soldered to one of the front and back contacts of a first cell and a trailing end soldered to a different contact of a second cell that is adjacent to the first cell. The foregoing objects, and other objects rendered obvious by the following detailed description, are achieved by providing a method of interconnecting in a string a plurality of solar cells each comprising a semiconductor substrate having front and back contacts bonded to its front and back surfaces respectively, and a solder coating on selected aligned areas of said front and back contacts, the preferred form of the method comprising the steps of: (a) feeding a first one of said cells to a cell receiving station so that a selected one of said front and back contacts of said first cell faces down and the other contact of said same cell faces up; (b) advancing said first cell from said cell receiving station to a cell-soldering station located downstream of said receiving station; (c) dispensing an elongate conductive ribbon from a supply roll thereof located upstream of said receiving station through said receiving station to said soldering station so that said ribbon overlies and is engaged with the solder coated areas of said other contact of said first cell and extends rearwardly from said soldering station through said receiving station; (d) releasably holding said ribbon against movement relative to said first cell in said soldering station; (e) cutting said ribbon immediately upstream of said receiving station so as to form a first cut-off section of ribbon having a leading end that overlies said other contact of said first cell and a trailing end that extends through said receiving station, and said ribbon on said supply roll has a new leading end located upstream of said receiving station; (f) soldering said leading end of said first cut-off section of ribbon to said other contact of said first cell; (g) feeding a second cell to said receiving station so that said second cell overlies and engages the trailing end of said first cut-off section of ribbon, said second cell being oriented the same as said first cell; (h) advancing said first cell out from said soldering station and simultaneously advancing said second cell to said soldering station; (i) advancing said new leading end of said ribbon on said supply roll through said receiving station to said soldering station so that ribbon from said roll overlies and is engaged with the solder coated areas of the upwardly facing contact of said second cell and extends rearwardly from said soldering station through said receiving station; (j) releasably holding the new leading end of said ribbon from said roll against movement relative to said second cell in said soldering station; (k) cutting said ribbon from said roll immediately upstream of said receiving station so as to provide a second cut-off section of ribbon having a leading end overlying the upwardly facing contact of said second cell and a trailing end that extends through said receiving station, and the ribbon on said roll has still another new leading end; (1) soldering the leading end of said second cut-off section of ribbon to said upwardly facing contact of said second cell and soldering the trailing end of said first cut-off section of ribbon to the other contact of said second cell; and (m) repeating selected ones of said steps in sequence so that sequentially additional cells are delivered to said receiving station and thereafter interconnected in a string by a plurality of cut-off sections of said ribbon, with each cut-off section of ribbon having one end soldered to one of said front and back contacts of one cell and its other end soldered to the opposite contact of an adjacent cell. It is to be appreciated that step (g) involving delivery of a new cell to the receiving station can occur immediately after step (e) or before or while soldering is being carried out according to step (f) . Also in the preferred embodiment hereinafter described, two ribbons are used to interconnect each pair of cells. The foregoing method results in several cells being interconnected electrically in series, preferably with a small gap between adjacent cells to allow for cell creep in a module when the latter undergoes expansion or contraction as a result of temperature excursions. The invention optionally and preferably includes crimping the ribbon between adjacent cells so as to form an offset in the ribbon that provides strain relief in the ribbon when the gap between adjacent cells undergoes contraction because of temperature or when the module of which the cell forms a part is subjected to forces that tend to cause flexing of the module. Although certain aspects of the method may be carried out by hand, e.g., the ribbon(s) and cells may be advanced and positioned manually, a particular 7387 „,- ^ , ™ „ PCT/IB94/00446 -11- advantage of the invention is that the method may be carried out automatically by machine. Accordingly the invention also includes provision of a machine as hereinafter described. Other features and advantages of the invention are described in or rendered obvious by the following detailed description of the invention which is to be considered together with the accompanying drawings. THE DRAWINGS Throughout the drawings, like reference numerals are utilized to identify like elements. It also should be understood that the drawings are intended to be illustrative only. The component mechanisms and control means are shown schematically because (1) they can be fabricated by ordinary persons skilled in the art without need for extensive experimentation and (2) they can take various forms. Also, for convenience and ease and clarity of illustration and description, components of the apparatus are neither shown to scale nor shown exactly in accordance with their relative proportions. Fig. 1 is a top plan view illustrating the front side of one form of solar cell that may be interconnected according to this invention; the anti-reflection coating is omitted from this view for convenience; Fig. 2 is a plan view of the rear side of the same solar cell; Fig. 3 is a cross-sectional view, taken along line 3-3 of Fig 1, showing the addition of solder paste coated on the bus bars of the front contact and the soldering pads of the rear contacts; Fig. 4 is a schematic plan view of a preferred form of machine embodying the present invention; Fig. 5 is a schematic side view in elevation of the same machine; Fig. 6 is a plan view on an enlarged scale of a portion of a preferred form of cell conveyor table for the machine illustrated in Figs. 4 and 5; Fig. 7 is a cross-sectional view of the conveyor table taken along line 7-7 of Fig. 6; Fig. 8 is a schematic view in elevation on an enlarged scale illustrating the top and bottom cutters in relation to the electrically conductive ribbons they are required to cut; Fig. 9 is a view in side elevation of a portion of the hot air soldering head; Fig. 10 is a bottom plan view on an enlarged scale showing details of the ribbon clamp mechanism at the soldering station; Fig. 11 is a schematic diagram illustrating the control system of the machine of Figs. 4-10; and Fig. 12 illustrates crimping of an electrical conductor according to this invention, with the ribbon being shown in solid lines for clarity. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The preferred embodiment of the invention hereinafter described provides a method and apparatus for efficiently and reliably interconnecting solar cells into strings of variable length by means of strips of solder-connected flexible electrical conductors that are crimped between cells so as to better assure reliable, long-lasting mechanical and electrical connections between adjacent cells. For the purpose of this invention it is to be understood also that the front and back contacts of each cell need to be pre-coated with a selected solder paste material. Also, although Figs. 1-3 illustrate solar cells of a type that the invention is particularly designed to interconnect, it is to be understood that the invention is not limited in its use to cells constructed exactly as shown in Figs. 1-3. Instead other solar cells may be interconnected by use of the present invention provided that the front and back contacts of the cells have solder coated areas aligned in a pattern designed to permit solder bonding of a conductive ribbon in the manner hereinafter described. Referring now to Figs. 1-3, the cell C shown in these figures comprises an EFG-grown polycrystalline silicon substrate in the form of a rectangular P-doped silicon sheet or wafer 2 that has been processed so as to have a shallow P-N junction 3 (Fig. 3) adjacent its front surface 5. Cell C also has on its front side a silver front contact 4 preferably in the form of a grid consisting of an array of narrow, elongate, parallel fingers 6 and two bus bars 8 that interconnect fingers 6. Additionally, a thin transparent silicon nitride AR coating 10 (Fig. 3) covers those portions of the front surface of the substrate that are not occupied by grid electrode 4. The rear side of cell 2 comprises a back or rear contact 12 (Fig. 2) that is made of aluminum metal and terminates short of the outer edges of the rectangular cell so as to have an uncoated margin portion 14 (Fig. 2) that extends along each side of the cell substrate coextensive with the periphery of the substrate, and also two rows of silver metal soldering pads 16 that fill apertures formed in the rear contact and are fused to the underlying solar cell substrate. Pads 16 overlap the rear contact around the periphery of the apertures in contact 12. Pads 16 are considered to be an integral part of the back contact. Although Fig. 2 shows eight soldering pads, it is to be understood that the number and spacing of the soldering pads may be varied and is not critical to this invention. Solar cells of the foregoing type are well known and, for example, are disclosed by PCT International Publication No. WO 92/02952, supra. The bus bars of the front contact and the silver soldering pads of the back or rear contact are coated with a solder paste as shown at 18 and 20 (Fig. 3) respectively which is then dried to form an adherent coating. The solder paste is applied to the bus bars directly in line with the soldering pads 16. Various solder pastes may be used, but it is preferred to use solder pastes having between about 96% tin/4% silver and 98% tin 2% silver, since such solder pastes are readily available commercially, are easily dispensed in small daubs using commercially available paste dispensing equipment, and are known to provide superior bond strengths. By way of example, one such commercially available solder paste is the "96% tin/4 % silver Xersin 2005" solder paste manufactured by Multicore Corp. of Westbury, N.Y. that is mentioned in U.S. Patent No. 5,074,920, issued 24 Dec. 1991 to R.C. Gonsiorawski et al for "Photovoltaic Cells With Improved Thermal Stability". That particular paste is a stable blend of pre-alloyed solder powder and a synthetic flux and has the advantage that it can be dispensed at ambient temperature, i.e., 25 degrees C. The solder may be applied by various means, but preferably it is applied by means of a solder paste dispensing machine like the one described and illustrated in co-pending U.S. application Ser. No. 08/191622, filed 4 February 1994 by David S. Harvey et al for "Machine and Method For Applying Solder Paste To Electronic Devices" (attorney Docket No. MTA-93) . To the extent that it may be necessary to do so, the information provided by said co-pending application Ser. No. 08/191622 is incorporated herein by reference. The flexible conductors used in the practice of this invention are in the form of flattened wire, i.e., a flat ribbon. The ribbon is made of copper or a copper alloy. Preferably the ribbon is made of dead soft CDAIOI copper alloy, with a hot tin dip coating on both sides measuring about 20 microinches thick and free of lead. The thickness and width of the ribbon may be varied but the ribbon should be substantially flat. Preferably a relatively thin ribbon is used, with the thickness being such as to assure that the ribbon will withstand handling and soldering conditions. For the purposes of this invention it is preferred that the ribbon have a width great enough to cover the daubs coated on the front and back contacts. Thus in the case of cells as shown in Figs. 1-3 measuring approximately 4" x 4" square with the uncoated margin 14 at each side of contact 12 having a width of about 0.060", bus bars 8 having a width of about 0.040", and silver soldering pads 16 being rectangular and measuring about 0.150 x 0.150" as viewed in Fig. 2, it is preferred to use CDAIOI copper alloy ribbon measuring about 0.004" thick and about 0.060" wide, with a camber not exceeding 0.250 inches over a length of 24 ft. By "camber" is meant the straightness of the ribbon along its length. Referring now Figs. 4-10, the invention also involves provision of a machine that preferably comprises a conveyor mechanism in the form of a flat and rigid or stiff elongate table 30 that is subdivided into a plurality of cell-receiving pockets or nests 32 (Fig. 6) by raised portions in the form of integral ridges 34 that extend above the upper surface 31 of the table. For convenience, ridges 34 and other details of table 30 and nests 32 are shown only in Figs. 6 and 7. Table 30 preferably is made of a metal, such as aluminum or stainless steel. Ridges 34 also serve as anvils for use in crimping the ribbons used to interconnect the cells. As seen in Fig. 7, ridges 34 comprise a relatively high center portion 34A and relatively low end portions 34B, all of which protrude above the level of upper surface 31. The upper edges of portions 34B are rounded to facilitate ribbon crimping as hereinafter described. Low end portions 34B are aligned with holes 36A-D hereinafter described. Each nest 32 also is provided with two parallel rows of relatively large evenly spaced holes 36A-D (Fig. 6) and also two parallel rows of relatively small evenly spaced holes that are aligned with holes 36A-D and are filled with tubular standoff members 37. Holes 36A-D are spaced so as to be aligned with those areas of the cell that are coated with solder paste. The standoff members 37 are made of a material that is a relatively poor heat conductor, e.g., Viton or a silicone rubber. Tubular members 37 are locked to table 30 and project above top table surface 31 by about 1/16" and function to support a cell in spaced relation to table 30, whereby to prevent the table from acting as a heat sink for a cell. As hereinafter described in greater detail, hollow standoff members 37 also function as means for applying suction to hold the flexible conductors in place in nests 32. Holes 36A-C are circular and identical in size, while holes 36D are keyshaped, comprising a circularly curved portion that is adjacent to and has the same radius of curvature as holes 36A-C, and a larger portion 39 adjacent a ridge 34 that is sized to accommodate a knife blade as hereinafter described. Cell aligning guides 38 and 40 (Figs. 6 and 7) are associated with each nest 32 (for convenience of illustration, guides 38 and 40 are omitted from Figs. 4 and 5) . Guide 38 is a flat plate that is fixed to the upper surface 31 of table 30 with its inner surface 41 extending parallel to the longitudinal axis of the table. Guide 40 comprises a flat plate having a transversely extending passageway in which is mounted a snubber member 42. The latter is biased by springs (not shown) in guide 40, so that normally it is retracted away from guide 38 so that its inner end surface is flush with the inner edge surface of plate 40. When member 42 is pressed inwardly toward guide 38 against its spring bias, its inner end surface will engage a cell that has been positioned in nest 32 and move the cell against the inner surface 41 of guide 38, thereby aligning the cell in the nest so that its two rows of silver soldering pads and its bus bars are aligned with the two rows of holes 36A-D and the end portions 34B of ridges 34. Each snubber member 42 is operated by a compliant pusher member 43A (Figs. 4 and 5) that is located to one side of table 30 at the cell-receiving station hereinafter described. Pusher member 43A is attached to and operated by movement of the operating shaft of a pneumatic actuator 43B (Figs. 4 and 11) that has its cylinder fixed (by means not shown) to a chassis 45 hereinafter described. Pusher 43A is normally retracted, but is movable toward table 30 by actuator 43B by an amount sufficient for it to engage whichever snubber member is in its path and move that snubber member a limited distance (e.g. 0.060") toward the corresponding guide 38, so as to move an intervening cell against the corresponding guide 38. Pusher member 43A is compliant; it pushes snubber member 42 with enough force to overcome the spring bias of member 42, but will allow the snubber member to stop when the intervening cell engages guide 38 even though actuator 43B is urging the pusher member further toward guide 38. Operation of actuator 43B is controlled by a pre-programmed programmable electronic controller 200 (Fig. 11) acting via an electrically operated air valve 202A. It is to be noted that air valve 202A and certain of the other air valves hereinafter identified are connected to deliver pressurized air (e.g., at about 80 psi) from an air compressor 125 (Fig. 11) to actuator 43B and other actuators hereinafter described, while other air valves hereinafter identified control flow of air to a vacuum pump 127 (Fig. 11) for suction purposes. According to the preferred mode of practicing this invention, when cells C are deposited in nests 32 they are oriented so that their bus bars 8 and their two rows of soldering pads 16 are in precise alignment with the two rows of holes 36A-D. Also, according to the preferred embodiment of the invention, the cells are delivered to and disposed in nests 32 so that their front grid contacts face down toward table 30 with bus bars 8 overlying and engaging conductive ribbons in nest 32, which in turn overlie and engage standoffs 37. Table 30 is mounted on a supporting chassis 45 (Figs. 4 and 5) for reciprocal movement along its longitudinal axis by means of a suitable drive represented schematically at 46. The latter may take various forms, but preferably it is a screw drive mechanism powered by a reversible d.c. servomotor (not shown) . Drive 46 is designed to move table 30 axially from left to right (as seen in Figs. 4 and 5) in predetermined increments equal to the length of each cell nest 32 measured along the table's longitudinal axis, and also to move the table rapidly from one to the other of two extreme positions as hereinafter described. As described hereinafter, table 30 transports cells from a cell receiving station (which is a predetermined location where a cell is placed in a nest 32) to a soldering station (which is a predetermined location where conductive ribbons are soldered to that cell) , and then out of the soldering station to a collecting station, where the resulting string of interconnected cells is removed for use in making multi-cell photovoltaic modules. The first extreme position of table 30 (the "table retracted position") is when the first-in-line nest 32 is located at the soldering station. The second extreme position is when the last-in-line nest 32 has been moved out of the soldering station to the collecting station. For each nest 32 located at the cell-receiving station, holes 36A are closest and holes 36D are farthest from the soldering station. Mounted above table 30 by support means (not shown) that are fixed to chassis 42 are two rotatable shafts 47A, 47B (Fig. 4) which support two rolls 48A, 48B of a flexible electrical conductor in the form of flat ribbons 50A, 50B respectively, e.g., copper ribbons. Shafts 47A, 47B are coupled to tension-responsive electrical motor ribbon supply drives represented schematically at 52A, 52B which are adapted to rotate the shafts in a counterclockwise direction (as viewed in Fig. 5) so as to cause the two rolls to rotate and thereby pay out ribbon as hereinafter described. Energization of drives 52A, 52B is controlled by controller 200 (Fig. 11) . Motor drives 52A, 52B include dancer mechanisms (not shown) that sense the tension on ribbons 50A, 50B and provide motor control signals that modulate operation of the motor drives so that the drives will maintain a substantially constant tension on those ribbons as they are being fed to the soldering station by the ribbon dispensing mechanism 60 hereinafter described. The forward ends of ribbons 50A, 50B are carried by a ribbon dispensing mechanism 60 that comprises a carriage 61 and a pair of hollow fingers 58A, 58B that are tied together as a unit and are pivotally mounted to the carriage. Carriage 61 is slidably supported (by a carriage support means not shown affixed to chassis 45) for reciprocal movement along a path that is above and extends parallel to the longitudinal axis of table 30 (as indicated by the dual direction arrows in Fig. 5) . Carriage 61 is connected to a selectively operable ribbon dispenser drive mechanism represented schematically at 62 that is mounted to a suitable stationary support means (not shown) carried by chassis 45. Drive mechanism 62 may take various forms. Preferably, it comprises an electric motor (not shown) that is adapted to move carriage 61 lengthwise of table 30 between a first retracted position (Figs. 4 and 5) and a second extended position, and also move it back to a third intermediate position, as hereinafter described. Operation of the electrical motor of drive mechanism 62 is controlled by controller 200 (see Fig. 11). Fingers 58A, 58B are pivotally mounted to carriage 61 so as to be pivotable on a horizontal axis 64 that extends at a right angle to the longitudinal axis of table 30 and also perpendicular to a plane that extends parallel to the plane of the paper on which Fig. 5 is drawn. Pivotal movement of fingers 58A, 58B on axis 64 is controlled by a dispenser pivot actuator means 63 which preferably is a pneumatic cylinder that is affixed to carriage 61 and has its operating shaft (not shown) mechanically coupled to the fingers via a pivot-type connection (not shown) so that reciprocal movement of that cylinder's operating shaft will pivot the fingers so as to move their free forward ends in an arc from a first raised position (Fig. 5) to a second lowered position, and vice versa. Operation of actuator mechanism 63 is controlled by an electrically operated air valve 202B. Dispensing mechanism 60 is mounted so that when fingers 58A, 58B are in their raised position (Figs. 5) , their forward ends are spaced a predetermined relatively large distance (e.g. 0.75") from the upper side of whatever dummy cell or solar cell C is positioned immediately below them on table 30, but when the fingers are in their second lowered position their forward ends are spaced a relatively small distance (e.g., 0.060") from the upper side of that same cell. Drive mechanism 62 and actuator 63 coact to cause dispensing fingers 58A, 58B to be moved lengthwise of table 30 back and forth between a first retracted position (Figs. 4 and 5) and a second extended position as hereinafter described, and also to cause fingers 58A, 58B to pivot on horizontal axis 64 from their raised position to their lowered position, or vice versa, as indicated by the double-headed arrow in Fig. 5. More specifically, as described hereinafter in greater detail, controller 200 is programmed to operate drive mechanism 62 and actuator 63 so as to cause dispensing mechanism 60 to undergo movement according to the following sequence starting with carriage 61 in its retracted position (Fig. 5) , and fingers 58A, 58B in their raised position (Fig. 5) : (1) carriage 61 is moved forward from its first retracted position to its second extended position and then stops, with the forward ends of fingers 58A, 58B in their raised position, (2) the forward ends of fingers 58A, 58B are lowered so as to bring them into their second lowered position close to table 30, (3) carriage 61 is moved back to an intermediate position that is located a short distance, e.g., 1", forward of its first retracted position, and remains there long enough to permit cutting of the ribbons as hereinafter described, (4) the carriage is moved further back to its first retracted position, and (5) the forward ends of fingers 58A, 58B are raised back to their original position (Figs. 4 and 5) . When carriage 61 is in its second extended position, fingers 58A, 58B extend over a nest 32 disposed in the cell-soldering station as hereinafter described both before and after their forward ends are lowered from their raised position. Still referring to Figs. 4 and 5, remotely controlled ribbon clamping means 66A, 66B are mounted to and carried by the forward ends of fingers 58A, 58B respectively. Although not shown in detail, it is to be understood that clamping means 66A, 66B are adapted to releasably clamp the ribbons 50A, 50B to fingers 58A, 58B respectively. Clamping means 66A, 66B may take various forms, but preferably they comprise two pneumatic actuators mounted to the fingers and having pressure-exerting members (not shown) attached to the outer ends of their operating shafts, with the latter shafts slidably extending into the interior spaces of fingers 58A, 58B through suitable holes (not shown) in the upper walls of those fingers. Clamping means 66A, 66B are adapted to move their pressure-exerting members into and out of ribbon clamping position relative to the fingers, with the ribbon clamping involving pressing the ribbons into tight engagement with the lower walls of the fingers. Operation of the air cylinders of clamping means 66A,B is controlled by an electrically operated air valve 202C. The latter operates in response to electrical control signals from controller 200 to cause the clamping means to move from a clamping state to a non-clamping state, or vice versa. As described hereinafter, clamping means 66A, 66B are operated so that (l) they clamp the ribbons to the fingers when carriage 61 and fingers 58A, 58B of ribbon dispensing mechanism 60 are in their retracted position (Figs. 4 and 5) and also while they are being moved forward to their extended position, (2) they continue to clamp the ribbons to the fingers as the forward ends of the extended fingers drop down close to table 30, (3) they release the ribbons immediately after the extended fingers have moved to their down position and before drive mechanism 62 starts to move carriage 61 back to its original retracted position, and (4) they are restored to their ribbon-clamping state after carriage 61 has reached its first retracted position and before or after (preferably after) the fingers are pivoted back to their original raised position. The delivery of cells to table 30 may be accomplished in various ways. Thus, for example, if the machine shown in Figs. 4 and 5 is located adjacent to a solder pasting machine, e.g., of the type disclosed in copending U.S. patent application Serial No. 08/191622, filed February 4, 1990 by David S. Harvey et al., for "Machine And Method For Applying Solder Paste To Electronic Devices" (Attorney Docket No. MTA-93) , a transfer mechanism (not shown) may be employed for transferring solder-coated cells directly from the solder pasting machine to nests 32 as each nest in turn is located at the positions referred to herein as the "receiving station". As an alternative approach, a cell loading platform 70 (Fig. 4) is secured to chassis 45 and defines a cell loading station. Loading platform 70 is located to one side of table 30 adjacent the cell receiving station and functions to receive and hold a cell that is to be delivered to table 30 for interconnection according to the invention. Cells may be fed to platform 70 by hand, or automatically by a cell feeder mechanism (not shown) which is adapted to transport cells that have been discharged from a solder pasting machine of the kind described in said co-pending U.S. application Ser. No. 08/191622, supra. 87 PCI7IB94/00446 -26- Associated with table 30 and platform 70 is a movable cell pickup and positioning assembly 74 (Figs. 4 and 5) that is mounted so as to be capable of both horizontal and vertical movement relative to chassis 45 and table 30. More particularly, assembly 74 is provided with a vertical slide in the form of a dove-tailed plate or bar 75 that is slidably received by a correspondingly shaped groove in a slide support 76 that in turn is supported for reciprocal horizontal movement by a support means (not shown) that is fixed to chassis 45. Vertical movement of assembly 74 relative to slide support 76 is controlled by a vertical drive mechanism 78 (Fig. 5) , while horizontal movement of slide support 76 and hence assembly 74 is controlled by a second drive means 79 (Fig. 4) that is affixed to the chassis and is coupled to slide support 76. Drive mechanism 78 preferably comprises an air cylinder (not shown) that is mounted on slide support 76 and has its operating shaft coupled to move assembly 74, whereby operation of the air cylinder will cause assembly 74 to move vertically relative to the slide support between a first elevated position (Fig. 5) which it occupies when the machine is at rest and a second lowered position close to table 30 wherein it can pick up or deposit cells as hereinafter described. Drive mechanism 79 preferably comprises an air cylinder that is adapted to move assembly 74 bidirectionally along a horizontal axis that extends parallel to the plane of table 30 but at a right angle to the longitudinal axis of that table, so that selectively drive mechanism 79 can position the assembly at a cell pickup position over platform 70 (Fig. 4) or a cell releasing (depositing) position directly over table 30. The area occupied by that portion of table 30 onto which cells are released (deposited) by assembly 74 forms the "cell receiving station" previously referred to. Operation of the air actuators that are used as drive mechanisms 78 and 79 is controlled by electrically operated air valves 202D and 202E that in turn are controlled by electrical signals from controller 200. The cell pickup and positioning assembly 74 comprises two or more depending parallel hollow tubes 82 (Fig. 5) having suction cups 84 on their bottom ends. Tubes 82 are arranged so as to facilitate suction gripping of a cell at spaced points along its upper surface for transfer movement as hereinafter described. The interior of assembly 74 is arranged to form an air manifold chamber 74A (Fig. 11) , and tubes 82 are connected to that manifold chamber. The latter in turn is connected to a vacuum pump 127 (Fig. 11) by suitable hose lines (not shown) via a suitable electrically operated flow control air valve 202F, so that on command suction may be applied to a cell via tubes 82 so as to hold the cell tight against suction cups 84. An upper cutting mechanism and a crimping device are mounted over table 30, with the upper cutting mechanism being located on the upstream side and the crimping device being located on the downstream side of the position occupied by cell pickup and positioning assembly 74 when the latter is positioned over table 30. These mechanisms may be mounted and arranged so as to be completely independent of one another and the cell pickup and positioning assembly. Alternatively, as shown, a cutter/crimper carriage 86 is positioned over table 30 and movably mounted to the chassis by suitable support means (not shown) . Carriage 86 extends lengthwise of table 30 as shown in Fig. 4 and also has two depending sections 87 and 88 that carry the upper cutting mechanism and the crimping mechanism. Vertical movement of cutter/crimper carriage 86 is achieved by an operating means 92 which preferably constitutes a pneumatic actuator having its air cylinder fixed to chassis 45 and its operating shaft (piston rod) attached to carriage 86. Operation of operating means 92 is controlled by an electrically operated air valve 202G. The upper cutting mechanism may take various forms but preferably it comprises a pair of upper cutter blades 90A,B (Fig. 8) that are attached to and depend from depending carriage section 87. Blades 90A,B are slightly wider than ribbons 50A,B. Vertical movement of carriage 86 moves the cutter blades between a first elevated non-cutting position (Fig. 5) and a second lowered cutting position wherein they engage the ribbons above the table. Also forming part of the machine is a lower (bottom) cutting mechanism 100, 102 (Figs. 5, 8) that is disposed below chassis 45 for cooperation with the upper cutter mechanism. The lower cutting mechanism may take various forms, but preferably it comprises*a pair of cutting blades 100 (Fig. 8) and operating means 102 to which blades 100 are attached, with operating means 102 being arranged to reciprocate blades 100 vertically relative to table 30. Bottom cutter blades 100 have notches 103 that have a width that is greater than the width of ribbons 50A, 50B, but narrower than upper cutter blades 90. The bottom (lower) cutter operating means 102 constitutes a pneumatic actuator which has its cylinder fixed with respect to chassis 45, with blades 100 being attached to and movable with the operating shaft of that air cylinder. Operation of bottom cutter operating means 102 is controlled by an electrically operated air valve 202H that is operated by signals from controller 200. The bottom cutter mechanism is arranged so that blades 100 can penetrate holes 36D in table 30 and overlap and slidingly engage upper cutter blades 90 as they are extended upwardly and downwardly respectively, so as to provide a shearing action on any ribbon that extends across their path of movement. Notches 103 are aligned so as to accommodate ribbons 50A, 50B. The notches help prevent displacement of the ribbons sidewise of table 30 during the cutting operation. Turning now to Figs. 6 and 8, the upper and lower cutter mechanisms are arranged so that the two upper cutter blades 90 and the two lower cutter blades 100 are aligned to penetrate the enlarged portions 39 of the two keyholes 36D of each nest 32 in table 30 when that nest is positioned in the receiving station, so that when the blades are extended in a cutting operation, blades 100 can move up far enough to be flush with or extend slightly above upper surface 31 (Fig. 6) of table 30 and will be overlapped by the downwardly moving blades 90. In this way, movement of blades 90 and 100 toward one another will cause severing of the ribbons 50A, 50B where they extend over and across keyholes 36D. The crimping device may take various forms. Preferably it comprises a pair of crimping blades 94A, B (Figs. 5, 7) that are adapted to engage and crimp the ribbons that extend across their path to shaft of movement. The bottom edges of blades 94 are rounded as shown in Fig. 12 and also by the shading in Fig. 7. Blades 94 are mounted to and extend radially from a horizontal shaft 103 that is rotatably mounted in the depending carriage section 88. Shaft 103 is connected for bidirectional rotation by an operating drive means which preferably consists of a pneumatic actuator 96 (Fig. 5) that has its cylinder mounted to carriage 86 and its operating rod connected to shaft 103 by a suitable crank-type linkage represented schematically by the dotted line 105. Operation of actuator 96 is controlled by an electrically operated air control valve 2021 (Fig. 11) . Actuator 96 and linkage 105 are adapted to rotate shaft 103 from a first position wherein blades 94A,B are disposed at an acute angle away from table 30 (Fig. 5) to a second position wherein the blades are rotated downward (counter¬ clockwise as viewed in Fig. 5) to a vertical (six o'clock) position. Downward movement of carriage 86 and counterclockwise movement of shaft 103 coact to cause blades 94A,B to engage the ribbons immediately adjacent to but upstream of the ridge 34 of the cell-receiving nest 32 that is positioned in the soldering station, and forces the engaged portions of the ribbons to bend to conform to the profile of ridge 34 and the adjacent top surface 31 of table 30, as shown schematically in Fig. 12, with the ribbons still overlying the standoffs 37 so that they can be held down by suction. Table drive means 46 is adapted to move the table in a stepwise manner relative to chassis 45, the table indexing the axial length of one nest 32 each time the drive means is stepped. Indexing movement of table 30 carries cells downstream from the cell-receiving station to the cell-soldering station, the latter constituting the region immediately adjacent to the receiving station and being characterized by the presence of a soldering head 120 (Figs. 4,5), a soldering station ribbon clamping mechanism 140, 142, a cell-raising mechanism 150, 152-154, and a cell cooling means 160 and 162A, 162B. Soldering head 120 is located directly above and in line with table 30 downstream of the crimping mechanism and is mounted by suitable means (not shown) attached to chassis 45 for reciprocal vertical movement from a first raised position (Fig. 5) to a second lowered position by a pneumatic actuator 121. Operation of actuator 121 is controlled by an electrically operated air valve 202J. Cell soldering head 120 utilizes hot air heating to accomplish the soldering. Accordingly it comprises (1) a chamber 122 (Fig. 9) that functions as a manifold for hot air. The air in chamber 122 is heated by one or more electrical heater elements (not shown but represented schematically at 123 in Fig. 11) . The heater elements are capable of heating the air to a temperature that is at least equal to, but preferably exceeds, the temperature required to soften the solder paste for soldering purposes. Soldering head 120 also comprises eight parallel hollow telescoping tube units 124 that depend from and communicate with manifold chamber 122. Tube units 124 are aligned with the eight solder-coated silver soldering pads 16 of a cell in the soldering station, whereby to direct hot air heated by the heater elements to the eight soldering pads. Soldering head manifold chamber 122 has a port to which is connected a flexible air feed conduit 126 (Fig. 9) that is connected by an electrically controlled air flow valve 202K to a compressor (or air blower) represented schematically at 125 (Fig. 11) that forces air through manifold chamber 122 to tube units 124. Each tube unit 124 comprise two telescoping tubes 125A, 125B (Fig. 9) made of an insulating material, with the bottom end of the lowermost tube being provided with a narrow extension in the form of a relatively thin split ring 134 made of a material that is heat resistant and will not solder bond to the cells or the ribbons. Preferably rings 134 are made of molybdenum and each has a flat portion 135 at the six-o-clock position to assure adequate contact with the ribbon disposed in its path. If desired, each tube unit 124 may include a coil spring (not shown) that acts on both tubes 125A, 125B so as to force tube 125B down into extended position relative to tube 125A. However, even without such springs, gravity tends to cause tubes 125B to an extended position relative to tube 125A. Tubes 125A, 125B and rings 134 are preferably made of a heat conductive metal such as brass or copper. Rings 134 engage the ribbons 50A, 50B that overlie the cell in the soldering station and thereby provide a vertical gap between the lower ends of the tube units and the ribbons, whereby hot air discharged from the tube units can flow unimpeded over the cell in the soldering station. Tube units 124 are arranged so that when the cell soldering mechanism is lowered by actuator 121 to its second lowered position, rings 134 will engage portions of the ribbons 58A, 58B that overlie a cell disposed on the conveyor table in the cell soldering station. In this connection it should be understood that the spacing between adjacent tube units 124 is identical to the spacing between adjacent silver soldering pads 16 and the spacing between the two rows of tube units 124 is the same as the spacing between the two bus bars 8 (also the spacing between the two rows of silver soldering pads 16) . When the soldering head is in its raised position, the tube units 124 are about 1 1/2" above the nest 32 in the soldering station. In that raised position, it is preferred that soldering head 120 be operated so as to preheat a cell in the soldering station as ribbons 50A, 50B are being delivered to it and also during the cutting and crimping operations, whereby to reduce the time required to cause the solder on the cells to melt and fuse the conductive ribbons 48A an 48B to a cell contact. The preheating also prevents thermal shock to the cells, as may occur if heating is applied only to the soldering sites during the solder bonding step. However, if risk of reduced productivity is acceptable, it may be feasible to discharge hot air from tube units 124 only when it is time for the soldering head to be lowered to solder ribbons to a cell. Ribbon clamping mechanism 140, 142 is located at the downstream end of the soldering station, as shown in Fig. 5. The soldering station ribbon-clamping mechanism may take various forms. Preferably it comprises a movable pressure-applying clamp member 140 (Fig. 5) having a pair of arms 140A, 140B (Fig. 10) that extend lengthwise over the table and have pressure pads or feet 141 at their free ends for applying pressure to the ribbons, and an actuator in the form of pneumatic cylinder 142 for moving member 140 between an elevated at-rest position (Fig. 5) and a lowered ribbon-clamping position hereinafter described. Cylinder 142 is fixed with respect to chassis 45. Operation of actuator 142 is controlled by an electrically operated air valve 202L (Fig. 11) . Clamp member 140 normally is in its elevated position relative to a cell in the loading station. When member 140 is moved down by operation of pneumatic cylinder 142, its pressure pads 141 engage the two ribbons that extend over the cell in the soldering station and presses those ribbons down against the cell. As shown in Figs. 5 and 10, the arms 140A, 140B are shaped so that pressure pads 141 are located between the first and second tube units 124A, 124B, i.e., the tube units that are located furthest from crimping mechanism 94, 96. Hence normal operating movement of pressure applying member 140 will not interfere with vertical movement of the soldering head, and vice versa. Associated with cell soldering head is a cell raising mechanism 150, 152, 154 that is disposed below conveyor table 30. The cell raising mechanism comprises a platform 150 that has affixed to it two parallel rows of upstanding telescoping pedestals 152, 153 that consist of upper solid cylindrical parts 152 that telescope within lower tubular parts 153. There are four pedestals in each row. Upper parts 152 are biased into extended position (Fig. 5) by spring means (not shown) disposed within lower tubular parts 153. At least upper parts 152, but preferably lower parts 153 as well, are made of an elastomer material that is a poor heat conductor, e.g., pedestals molded of a silicone rubber. The size and spacing of pedestals 152, 153 is such that each pedestal will protrude through a different one of holes 36A-D in table 30 when platform 150 is raised. The outer diameter (o.d.) of each upper part 152 is sized so that there is a gap between it and the hole 36A-D into which it is inserted by movement of platform 150. The pedestals are arranged to engage the ribbons underlying the cell in the loading station in line with the solder-coated areas of the cell and to raise the ribbons and cell up away from the nest. In so doing, the pedestals serve to balance the load on each cell subsequently exerted by soldering heat tubes 125B and split rings 134. Cell raising platform 150 is coupled to and raised and lowered by a pneumatic platform raising and lowering drive means 154. The latter preferably consists of a pneumatic actuator having its cylinder secured to chassis 45 and its operating shaft attached to and supporting platform 150. Drive means 154 is arranged so that when it raises the platform, the pedestals will lift a cell in the nest below the soldering head a short distance above table 30, e.g., .030 - .060", whereby to allow air to flow under the raised cell. Operation of actuator 154 is controlled by an electrically controlled air valve 202M. Also associated with the apparatus shown in Figs. 4 and 5 is a cell cooling mechanism in the form of a cooling air manifold 160. The latter is attached to and moves up and down with soldering head 120. Also forming part of the cell cooling mechanism is a cooling air distributing assembly that is attached to and movable with manifold 160 and comprises a pair of pipes 162A, 162B that extend below the hot air manifold 122 lengthwise of table 30 alongside the two rows of tube units 124. Preferably, pipes 162A, 162B are further apart from one another than the distance between corresponding side edges of the cell in the soldering station. Cooling air manifold 160 is coupled by appropriate flexible conduit means (not shown) to the compressor (or blower) 125 (Fig. 11) via an electrically operated control valve 202N. Each of the pipes 162A, 162B has a plurality of air discharge nozzles 163 that are located so as to discharge cooling air in a downward and inward direction, so as to cause air to flow over a cell that has been lifted in the soldering station by the cell-raising mechanism previously described. The cell cooling means 160, 162 is operated by valve 202N in sequence with the hot air soldering head 120, for the purpose of solidifying the solder that has been melted by the soldering head, whereby to produce a strong solder bond between the ribbons 58A, 50B and a cell that is disposed in the soldering station. It is to be appreciated that after a cell has been soldered to ribbons as hereinafter described, table 30 is indexed so as to move the soldered cell downstream out of the soldering station for the purpose of making room for the next successive cell which is to be soldered to sections of the ribbons 50A, 50B. The area located downstream of the soldering station is considered to be a collecting station, where cells that have been strung together according to the invention may be removed from table 30 and used in the construction of modules comprising one or more strings of cells. The machine also includes two vacuum plenums 170 and 172 that are fixed to chassis 45 and are adapted to make a substantially air-tight sliding engagement with the adjacent side surface of table 30, the latter having associated with each nest 32 a plurality of internal passageways, represented schematically by the dotted lines 174 in Fig. 6, that are connected at their inner ends with the bottom ends of standoff members 37, whereby suction may be applied to the interior space of the standoff members. The outer ends of internal passageways 174 make a sliding substantially air-tight engagement with ports (not shown) in plenums 170 and 172. Plenums 170 and 172 are located so that plenum 170 makes a sliding seal vacuum connection with the outer ends of passageways 174 of the nest located at the receiving station, while simultaneously plenum 172 makes a sliding vacuum connection with the outer ends of passageways 174 of the nest located at the soldering station, whereby a suction force can be applied to the copper ribbons 50A, 50B disposed on standoff members 37 in the receiving station and then again on the same copper ribbons in the soldering station. Plenums 170 and 172 are connected by conduit means (not shown) to vacuum pump 127 via suitable electrically operated air flow control valve means 2020 and 202P respectively (Fig. 11). The machine further includes a cooling air manifold or plenum 180 that is mounted to the upper side of pedestal platform 150 between the two rows of pedestals and is adapted to engage and make a tight seal with the underside of table 30 when the platform is raised. Table 30 has a plurality of inclined internal passageways, represented schematically at 184 in Fig. 6. The upper ends of passageway 184 intersect holes 36A-D, while their lower ends terminate in openings 185 in the bottom surface of table 30. Plenum 180 has an elongate opening at its top end (represented by the dotted line image 187 in Fig. 6) that is sized so as to encompass the two rows of bottom openings 185 of the nest 32 in the soldering station. When pedestal platform 180 is raised into sealing engagement with table 30, cooling air can be delivered into passageways 184 from plenum 180. The latter is connected by conduit means (not shown) to blower 125 via an air control valve 202Q that is controlled by controller 200. When valve 202Q is opened, cooling air is delivered into plenum 180, and that air flows from the discharge port 187 of the plenum through openings 185 and passages 184 (Fig. 6) into holes 36A-D, and some of this air flows upwardly to help cool the cell that is supported by pedestals 152, 153. In Fig. 11, which illustrates the control system that controls the mode of operation of the machine shown schematically in Figs. 4 and 5, electrical signal connections are shown in full lines and pneumatic conduits are shown in broken lines. The heart of the control system of Fig. 11 is the programmable controller 200 which may take various forms but preferably comprises a digital computer that is programmed by suitable software to provide the mode of operation hereinafter described. The mode of operation of the machine hereinafter described exemplifies a preferred form of the method of the present invention. For convenience of description, assume that with the machine in its initial at-rest position, i.e., before the machine has been actuated to commence its operating cycle, all of the nests 32 are empty, except for the first-in-line nest which is occupied by a dummy cell. Preferably the dummy cell is a solar cell that has no solder paste on its front and back contacts. Alternatively a non-cell blank, e.g., a rectangular EFG-grown silicon substrate that is substantially identical in size to a cell C, may be used as the dummy cell. Also a first cell C, precoated on both sides with solder paste as previously described, has been deposited on platform 70 with its front grid contact facing down, and table 30 is in its retracted position, which is the position wherein the first-in-line nest 32 containing the dummy cell is located in the soldering station. At the same time, the ribbon-dispensing mechanism 60 is in its first (retracted) position (Figs. 4 and 5) with actuator 63 set so as to hold the forward ends of the fingers 58A, 58B in raised position (Fig. 5) and clamps 66A, 66B set so as to clamp ribbons 50A, 50B to the fingers. Also, the cell pickup and positioning assembly 74 is in its elevated position and is displaced horizontally from table 30 so as to be located directly above cell-receiving platform 70, compressor 125 and vacuum pump 127 are operating, valves 2020 and 202P are set so as to provide a suction force to plenums 170 and 172, air valve 202F is set to prevent application of suction to manifold 74A and tubes 82, valve 202J is set so that hot air soldering head 120 is in its raised position over table 30, valve 202M is set so that cell raising platform 150 is in its "down" position, valve 202L is set so that clamping member 140 is in its raised non-clamping position, valves 202G and 202H are set so that cutter/crimper carriage 86 and lower cutter means 100, 102 are in their raised and lowered positions respectively, and valve 2021 is set so that crimping blades 94A,B are in their inclined non-crimping position, all as illustrated in Figs. 4 and 5. Additionally, valve 202K is set so that hot air is being discharged by soldering head tube units 124, and valves 202N and 202Q are set so that no cooling air is being discharged by cell cooling air pipes 162A,B and no cooling air is being injected into holes 36A-D from plenum 180. Operation of the machine is initiated by actuating a "start cycle" switch (not shown) , which activates controller 200 so that the latter will begin to execute its machine control program. Controller 200 immediately energizes ribbon supply motors 52A, 52B and activates ribbon dispenser drive 62 so as to cause dispenser 60 to advance ribbons 50A, 50B forwardly over the dummy cell in the soldering station. In this connection, it is to be noted that when ribbon dispenser carriage 61 is in its retracted position, the forward ends of fingers 58A, B are located about 1.0 -1.5" upstream of the cutting plane of the ribbon cutting mechanism hereinafter described and ribbons 50A, 50B project forwardly beyond the tips of the fingers 58A, 58B by approximately 0.75". The extent of the forward movement of carriage 61 by operation of drive 62 is such that the fingers are stopped when the forward end edges of the ribbons are about 0.25" short of the downstream edge of the dummy cell in the soldering station. Controller 200 stops drive 62 with the fingers in their extended positions over the dummy cell. As soon as the ribbons have been positioned over the dummy cell, actuator 63 is operated to pivot the forward ends of fingers 58A, 58B, downwardly far enough to cause the forward tips of the ribbons to engage the dummy cell downstream of where the feet 141 of soldering station ribbon clamp member 140 will engage them. As soon as the ribbons have been extended forward over the dummy cell, controller 200 operates valve 202L so as to cause ribbon-to-cell clamp actuator 142 to lower member 140 far enough for its pressure pads 141 to engage the ribbons 50A, 50B at points located between the first and second (counting from right to left in Fig. 5) hot air tube units 124A and 124B (see Fig. 10) and clamp the ribbons to the dummy cell. As soon as the pressure feet 141 have engaged ribbons 50A, 50B, controller 200 reactivates drive 62 so as to cause carriage 61 to move back out from the soldering station to an intermediate position short of its fully retracted position. However, before this action occurs, the controller operates valve 202C so as to cause clamps 66A, 66B to be released, with the result that as the carriage 61 is moved back to its intermediate position, the fingers 58A, 58B will slide relative to ribbons 50A, 50B respectively due to the fact that the ribbons are held against movement by action of soldering station clamping member 140. When carriage 61 has reached its intermediate position (the forward ends of fingers 58A,B are still in lowered position), ribbons 50A,B extend back from the dummy cell over and lengthwise of the next-in-line (second) nest 32 located at the receiving station and also over the ridge 34 on the upstream side of that nest. The suction applied via plenum 170 draws the ribbons down and holds them in engagement with standoff members 37 of that next-in-line (second) nest 32 located in the receiving station. Next controller 200 acts through valves 202G and 202H to operate cutter/crimper carriage actuator 92 and lower cutter actuator 102 simultaneously so as to cause cutter blades 90 and 100 to move and coact to cut the ribbons immediately downstream of the ridge 34 on the upstream end of the nest 32 in the receiving station, thereby creating new leading ends of the ribbons carried by the fingers that project forwardly of the fingers by about 0.75 inch. Immediately after the ribbons have been cut, valve 202H is operated so as to cause the lower cutter to drop down to its original position, and valve 2021 is operated so as to rapidly cause crimper device actuator 96 to reciprocate crimper blades 94A,B, whereby the latter are caused to rotate down (counterclockwise as viewed in Fig. 5) to engage the ribbons 50A,B immediately upstream of the ridge 34 of the nest 32 in the soldering station, causing the ribbons to be crimped around that ridge 34 as shown in Fig. 12, and then to rotate back up to their original inclined position. The crimped ribbons remain in their crimped state after blades 94A, 94B are returned to their original inclined position. Virtually simultaneously with operation of the crimper mechanism, but preferably immediately afterward, controller 200 (a) operates valve 202C so as to cause clamps 66A,B to clamp the new leading ends of ribbons 50A,B (i.e., the ends of the ribbons on rolls 48A,B) to fingers 58A,B and (b) also causes drive 62 to move carriage 61 back to its first retracted position. Also virtually simultaneously with return movement of carriage 61 from its intermediate position to its first retracted position, controller 200 operates valve 202B so as to cause actuator 63 to restore the fingers to their original raised positions. Controller 200 then causes actuator 92 to raise the cutter/crimper carriage 86 back to its elevated at-rest position. Then controller 200 operates the control valves 202D and 202E for cell pickup and positioning actuators 78 and 79 and vacuum valve 202F so that the following operations occur in sequence, (a) cell pickup and positioning assembly 74 moves down and contacts the first cell C on platform 70, (b) assembly 74 picks that cell up off of platform 70 by suction, (c) assembly 74 moves horizontally so as to place that cell directly over table 30, (d) assembly 74 lowers the new cell into overlying engagement with the cut sections of ribbons 50A,B that extend across the nest 32 that is in the receiving station, i.e., the second-in-line nest, (e) assembly 74 releases that cell so that it is supported by table 30, and (f) assembly 74 moves up away from table 30 and then back over platform 70. Then clamping member 140 is raised away from the dummy cell and drive 46 is operated to advance table 30 (i.e., move the table left to right in Fig. 4) far enough to (1) move the dummy cell out of the soldering station, (2) place that first cell C in the soldering station directly below soldering head 120, and (3) place the next (3rd) nest 32 in the receiving station. At this point, the two lengths of ribbons 50A,50B previously cut by cutter mechanism 90, 92 and 100, 102 have their trailing ends underlying the first cell C in the soldering station and their leading ends overlying but not attached to the dummy cell which now is located downstream of the soldering station. Thereafter controller causes the machine to repeat certain of the steps previously described, starting with activation of motors 52A, 52B and ribbon dispenser drive 62 so as to cause dispenser 60 to advance ribbons 50A, 50B forwardly over the first cell C in the soldering station. As soon as the ribbons have been positioned over the first cell C, actuator 63 is operated to pivot the forward ends of fingers 58A, 58B, downwardly far enough to cause the forward tips of the ribbons 50A, 50B to engage the first cell C downstream of where the feet 141 of soldering station ribbon clamp member 140 will engage them. At this point, the tips of the ribbons are about 0.25" short of the leading (downstream) edge of that first cell C. Immediately thereafter actuator 142 is operated to lower member 140 far enough for its pressure pads 141 to engage the ribbons 50A, 50B, at points located directly below the space between the first and second hot air tube units 124A and 124B (Fig. 10) and clamp the ribbons to the new cell. As soon as the pressure feet 141 have engaged ribbons 50A, 50B, drive 62 is reactivated to cause dispenser carriage 61 to move back out from the soldering station to its intermediate position short of its first (fully retracted) position. However, before this action occurs, clamps 66A, 66B are released, with the result that as carriage 61 moves rearwardly back toward its original retracted position, fingers 58A, 58B will slide relative to ribbons 50A, 50B due to the fact that the ribbons are held against movement by action of soldering station clamping member 140. Dispenser carriage 61 is stopped in its intermediate position, at which point in time ribbons 58A, B extend back from the new cell C in the soldering station across the next-in-line nest 32, i.e., the third nest, located at the receiving station, and suction applied to that next-in-line nest by plenum 170 via valve 202O acts to hold the ribbons down against hollow standoffs 37 of that next-in-line (third) nest 32. Then actuators 92 and 102 are operated as previously described so as to cause cutter blades 90 and 100 to cut the ribbons immediately downstream of the ridge 34 on the upstream end of the third nest 32 located at the receiving station. Immediately thereafter, the lower cutter is dropped down to its original position and crimper device actuator 96 is again operated as previously described to cause the crimper blades 94A, 94B to engage the ribbons immediately upstream of the ridge 34 on the upstream end of the nest 32 in the soldering station, causing the ribbons to be crimped around that ridge as shown in Fig. 12. The crimper blades are then rotated back to their original inclined position. Virtually simultaneously with operation of the crimper mechanism, but preferably immediately afterward, controller 200 operates valve 202C so as to cause clamps 66A, 66B to clamp the new leading ends of ribbons 50A, 50B (i.e., the ends of the ribbons on rolls 48A, 48B) to fingers 58A, 58B and also operates drive 62 to move carriage 61 back to its first retracted position. Also virtually simultaneously with that return direction movement of carriage 61, controller 200 operates valve 202B so as to cause actuator 63 to pivot the fingers back to their original raised position. Controller 200 then cause actuator 92 to raise the cutter/crimper carriage 86 back to its elevated at-rest position. As carriage 86 is being raised, or immediately thereafter, controller 200 (a) operates valve 202M so as to cause actuator 154 to lift the first cell C up so that it is spaced from table 30 by about 0.60", and (b) operates valve 202J so as to cause actuator 121 to lower soldering head 120 far enough to cause the ring members 134 on the bottom ends of telescoping tubes 124 to engage the ribbons that overlie the cell. Preferably, but not necessarily, controller 200 also operates valve 202L so as to cause actuator 142 to raise clamp member 140 out of clamping position as soon as ring members 134 engage the ribbons that overlie the first cell C. Controller 200 causes actuator 121 to keep the soldering head in its "down" heating position (i.e., with hot air flowing out of tubes 124) long enough to melt the solder paste on the solder-coated cell, e.g. , about 2-3 seconds. Then the controller (a) operates valve 202K so as to shut off the flow of hot air to tubes 124 and (b) operates valves 202N and 202Q so as to cause cooling air, e.g. , air at 25°C, to flow out of pipes 162A,B and through passageways 184 to holes 36A-D so as to cool the first cell in the soldering station. Valves 202N and 202Q are operated to turn off the flow of cooling air after about 3 seconds. Then controller 200 operates valve 202J so as to cause actuator 121 to raise the soldering head back to its original elevated position, and also operates valve 202M so as to cause actuator 154 to lower platform 150 back to its original "down" position, whereby to move the cell C back down onto table 30. After the soldering head has been raised and platform 150 has been lowered as described above, controller 200 again operates valve 202K to cause hot air to be discharged from tubes 124. This action may be delayed until a new cell C has been advanced to the soldering station. Thereafter the machine repeats the foregoing sequence of steps, starting with operation of valves 202D and 202E for cell pickup and positioning actuators 78 and 79 and also vacuum valve 202F so as to cause the cell pickup and platform assembly to carry out the steps (a) to (f) described above, whereby a second new cell C is deposited in the third nest 32 located in the receiving station. Then table 30 is indexed to move that third cell to the soldering station, after which the machine repeats the ribbon dispensing, cutting, crimping, and soldering steps as already described. It is to be appreciated that each cell C undergoes preheating by the soldering head as soon as that cell has been advanced to the soldering station, even though that head is still in its raised position. When the soldering head is lowered to soldering position, the hot air discharged from tubes 124 is adequate to heat the new cell, the ribbons, and the solder paste enough to melt the solder paste on the upper and lower surfaces of the cell, as is required in order to solder the trailing ends of the ribbons underlying the new cell and the leading ends of the ribbons overlying the new cell. Because the cell undergoes pre-heating by the soldering head while that head is still in its raised position, the soldering head needs to be engaged with the cell only a short time, e.g., about 3 seconds, in order to cause the solder to melt and wet the ribbons That solder is re-solidified rapidly by the cooling air delivered by pipes 162A,B and passageways 184. In the preferred embodiment of the invention, table 30 has twenty nests 34, and after the ribbons extending lengthwise of the 20th nest (i.e., the sections of ribbon that are soldered to the upper side of the 18th cell in the 19th nest) have been cut and crimped, the controller immediately terminates the cycle of operation of the machine and then immediately causes drive 46 to rapidly move and the entire table 30 downstream of the soldering head to an area (the "collecting station") where the string of interconnected cells in the second through nineteenth nests may be removed from table 30. At this point, table movement is stopped and the machine is disabled by controller 200, so as to afford the operator unlimited time to remove the string of eighteen interconnected cells from table 30. The dummy cell is retained in the first nest for use again in the stringing process. After the string of cells has been removed from table 30, the operator restarts operation 87 P - IB94/00446 -50- of controller 200, which immediately causes conveyor drive 46 to rapidly return the table to its original retracted (starting) position ready for stringing together another group of cells according to the sequence of steps described above. In this starting position, the dummy cell is again at the soldering station. Commencement of a new cycle of operation occurs when the operator again actuates the "start cycle" switch referred to above. The apparatus just described offers a number of advantages. For one thing, it facilitates the interconnection of photovoltaic solar cells by means of strips of a flexible electrical conductor (e.g., ribbons 50A, 50B) that are equal in length and applied precisely to predetermined areas of the cells. Also the cells are strung together with gaps of pre¬ determined size between adjacent cells. Another advantage is that the conductors are advanced from supply rolls in a manner that avoids breakage, stress or fatigue, yet allows the conductors to make flat contact with the cells so as to facilitate soldering the conductors to the cells. Providing stress-relieving crimps in the ribbon is advantageous with respect to the integrity of the string of interconnected cells, and is particularly important in the use of polycrystalline cells having rough edges. For effective strain relief it is preferred that each crimp be formed with a substantial distance between the center portion of the crimp and the two points where the ribbon is bent into planar alignment with the two connected cells. A further advantage is that the conductor strips are soldered by means that do not exert undue mechanical pressure on the cells, whereby damage to the cells is avoided. Another important advantage is that soldering is conducted by hot air which achieves melting of the solder without subjecting the cells to excessive mechanical strain. Also, the method advantageously employs preheating the solder-coated cells so as to accelerate the hot air soldering step and avoid or reduce thermal shock and non-uniformities. The precise locations of the hot air delivery tubes 124 and the pedestals 152, 153 in relation to the solder coated areas of a cell in the soldering station assures that the ribbons will be reliably soldered to the front and rear cell contacts. The method and means by which the conductors are transported into overlying relation with the cells is also advantageous. Avoiding application of any mechanical force to the cells while positioning the conductors so that they overlie the cells, or while conducting the soldering operation, is important since the silicon cells, which typically have a thickness in the range of 0.012 to 0.018 inch if they are made from polycrystalline EFG-grown substrates, are quite brittle and hence prone to breakage. An important productivity advantage is afforded by the invention because of the fact that conductor strips are soldered simultaneously to multiple sites on both sides of a solar cell. Hence still another advantage is that the invention provides a novel and reliable apparatus and method for soldering flexible conductors to multiple sites on the front and/or back contact of a solar cell simultaneously. Another advantage results from the fact that it is able to make use of a rigid conveyor table 30 that is made of metal, and hence highly conductive to heat, since the standoff members 37 keep the cells out of direct contact with the table and thus prevent the table from acting as a massive heat sink. The soldering process is further enhanced as a result of having a cell raising means that keeps the conductors underlying the raised cell in contact with the solder coated areas of the cell and out of contact with table 30 when solder cooling is being conducted. A further significant advantage is that the apparatus is relatively simple to construct and hence is reliable in its operation. The invention also is beneficial because it may be adapted for use with means for automatically feeding pre-soldered cells from a solder paste applying machine, e.g., a machine of the type disclosed in said U.S. patent application Ser. No. 08/191,622, to the cell input platform 70 or directly into nests 32 of table 30. Another important advantage of the invention is that it may be modified in various ways. Thus the number of soldering sites on each cell surface may be varied. Also, for example, the illustrated apparatus and method may be modified to utilize two separate soldering heads, one for soldering conductor strips to the front contact and the other for soldering the same strips to the back contact. In such a modification, the soldering to the front and back contacts may be accomplished simultaneously or one after the other. In the latter case, it is immaterial whether the strips are bonded first to the front or back contacts. The apparatus also may be provided with (a) sensor means (not shown) to determine when the supply of flexible conductors 50A, 50B is exhausted or when a cell is not available for transfer to a nest in table 30, or if one or the other of the two conductor strips has not been properly positioned, and using the output signals from those sensors to stop operation of the machine. Similar sensors may be used to prevent operation of the machine in the event that other operating components of the machine, e.g., the cell pickup and positioning assembly, and the cutting, crimping or hot air heating means, do not function as intended. The invention also may be modified by substituting a variety of conventional components, such as electrical motors and appropriate linkages as the drives or actuators for the various operating subassemblies, e.g., cell pickup and positioning assembly 74, ribbon dispenser 60, soldering head 120, the cutting and crimping mechanisms. Of course, the method may be practiced without using a machine having all of the features of the machines described hereinabove. Thus, for example, the cell pickup and positioning assembly 74 and platform 70 may be omitted, and cells may be delivered by hand to each nest that is located at the receiving station. It also is to be understood that the order of certain of the various steps of the method and apparatus described above may be varied without departing from the principles of the invention. Thus, for example, when carriage 61 is retracted back from the soldering station, the clamps 66A, 66B may be restored to ribbon-clamping position before or after the ribbon cutters are operated. Also, although ribbon clamp member 140 is normally released from its ribbon-clamping position as soon as the ribbons are engaged by rings 134 of the soldering head, it may be released slightly before the rings engage the ribbons or may be moved to non-clamping position after the soldering head has returned to its at-rest elevated position. It is to be appreciated that although the crimping means 94, 96 may be eliminated, their use is preferred since best results are achieved when the ribbons are crimped as shown. Also the form of the crimping means may be varied so long as it causes an offset crimp to be formed in the ribbons that is adequate for strain relief purposes. Of course, the number of conductive ribbons soldered to each cell may be varied; thus the front and back contacts may have only one rather than two conductive ribbons thereto. A further modification of the invention involves applying a solder coating to the ribbons rather than to the cell contacts. Also, the soldering head may be modified to employ some other form of heating means for heating and reflowing the daubs of solder on the cells in the soldering station. Still another obvious modification is to position the cells in nest 32 so that their rear contacts face down toward table 30. Other advantages and modifications of the invention will be obvious to persons skilled in the art.

Claims: WHAT IS CLAIMED IS: 1. Method of interconnecting solar cells in a string, each cell comprising a substrate having substantially flat first and second opposite surfaces, first and second contacts bonded to said first and second surfaces respectively, and a solder coating on selected aligned areas of said first and second contacts, said method comprising the steps of: (a) delivering a first cell to a receiving station so that said first contact is facing down; (b) advancing said first cell from said receiving station to a soldering station located downstream of said cell receiving station; (c) advancing the leading end of at least one elongate electrically-conductive ribbon from a roll thereof located upstream of said receiving station through said receiving station to said soldering station so that said ribbon overlies and is engaged with solder-coated areas of said second contact of said first cell and extends rearwardly from said soldering station through said receiving station; (d) releasably holding said leading end of said ribbon against movement relative to said first cell in said soldering station; (e) cutting said ribbon immediately upstream of said receiving station so that the cut-off section of ribbon has a trailing end that extends through said receiving station and the ribbon on said roll has a new leading end; (f) soldering said leading end of said cut-off section to said second contact of said first cell; (g) feeding a second like cell from said loading station to said receiving station so that said first contact of said second cell faces down and engages the trailing end of said cut-off section of ribbon; (h) moving said first cell downstream out of said soldering station and simultaneously advancing said second cell to said soldering station; (i) advancing the new leading end of said ribbon from said roll through said receiving station to said soldering station so that said new leading end overlies and is engaged with the solder coated areas of said second contact of said second cell and extends rearwardly from said soldering station through said receiving station; (j) releasably holding said new leading end of said ribbon against movement relative to said second cell; (k) cutting said ribbon immediately upstream of said receiving station so as to provide another cut-off section of ribbon having a trailing end extending through said receiving station and the ribbon on said roll has another new leading end; (1) soldering the leading end of said another cut-off section of ribbon to said second contact of said second cell and soldering the trailing end of said first cut-off section to said first contact of said second cell; and (m) repeating said steps in sequence starting with step (g) so as to interconnect a plurality of cells with said ribbon in an electrical series connection. 2. Method according to claim 1 further including the step of crimping said cut-off section of ribbon at a point immediately upstream of said soldering station before a new cell is advanced to said receiving station. 3. Method according to claim 2 wherein said cutting and crimping are conducted before step (g) . 4. Method according to claim 1 wherein soldering is conducted by hot air heating of the ribbon and cell in the soldering station. 5. Method according to claim 4 wherein said hot air heating is followed by directing a cool air stream at the cell in said soldering station. 6. Method according to claim 1 wherein said cells are conveyed from said receiving station to said soldering station and then downstream of said soldering station by a conveyor having a series of nests each sized to receive one cell. 7. Method according to claim 6 further including the step of raising each cell up off of said conveyor in said soldering station so as to facilitate heat transfer to said cell. 8. Method according to claim 6 wherein cells are fed to said receiving station from a loading station located to one side of the path of movement of said conveyor . 9. Method according to claim 8 wherein cells are fed from said loading station to said receiving station by a cell feeder mechanism that employs suction to lift the cells up from the loading station and carry them to said receiving station. 10. Method according to claim 1 wherein said ribbon is paid off of said roll and transported to said soldering station by a mechanical ribbon feeder that is reciprocated between a first retracted position upstream of said receiving station and a second position in which it extends into said soldering station. 11. Method according to claim 1 wherein each of said first and second contacts has a solder coating on each of a first series of aligned areas thereof and also on a second series of aligned areas thereof, and further wherein two electrically-conductive ribbons are soldered to said first and second series of aligned areas of said first contact and two other ribbons are soldered to said first and second series of aligned areas of said second contact. 12. Apparatus for interconnecting photovoltaic solar cells in a string using a flexible pre-tinned conductor, each cell comprising a substrate having substantially flat first and second opposite surfaces, first and second contacts bonded to said first and second surfaces respectively, and a solder coating on selected areas of each contact, said apparatus comprising: (a) a solar cell loading station for receiving and temporarily holding cells to be interconnected; (b) conveyor means for receiving cells from said loading station at a first receiving station and advancing them one at a time in series along a selected axis from said receiving station to a soldering station located downstream of said receiving station; (c) cell feeder means for feeding cells one at a time from said loading station to said conveyor means at said receiving station; (d) drive means for intermittently moving said conveyor means so as to advance a cell carried thereby stepwise from said receiving station to said soldering station and then to said collecting station; (e) supply-supporting means for holding a supply roll of an elongate conductor in the form of a pre-tinned metal ribbon; (f) ribbon feeder means mounted for reciprocal movement parallel to said axis between a first position located upstream of said receiving station and a second position over a cell positioned in said soldering station, said ribbon feeder means including first clamp means for releasably holding the leading end of said ribbon as said feeder means moves from said first position to said second position and for releasing said ribbon so as to permit relative movement therebetween as said ribbon feeder means moves back toward said first position, whereby if the leading end of said ribbon is restrained at said soldering station as said feeder means is moved back toward said first position from said second position, said ribbon will be dispensed so as to extend back from said soldering station over said conveyor means through said receiving station to said feeder means; (g) second clamp means for releasably clamping the leading end of said ribbon to a cell disposed at said soldering station; (h) soldering means at said soldering station for soldering the leading end of said ribbon to said first contact and the trailing end of another ribbon to said second contact simultaneously; (i) cutter means for cutting said ribbon immediately upstream of said receiving station; (j) means for operating the foregoing means in sequence and repetitively so that the following steps are carried out: (1) the leading end of the ribbon from said roll is advanced by said ribbon feeder means so as to overlie a first cell in the soldering station and extend through the receiving station, (2) the leading end of the ribbon is held against movement relative to said first cell and the ribbon carrier is moved back out of the soldering station, so that an exposed section of said ribbon extends from said soldering station through said receiving station, (3) the ribbon is cut so as to create a cut-off section of ribbon that extends back across said first cell in the soldering station through the receiving station, (4) the leading end of the cut-off section of ribbon is soldered to said first cell in the soldering station, (5) a second cell is delivered to the receiving station so that it overlies the trailing end of the cut-off section of ribbon, (6) the conveyor means, .moves the first cell out of the soldering station and moves the second cell to the soldering station, and (7) then the process is repeated starting with advancement of the new leading end of the ribbon so that it overlies the second cell in the soldering station, whereby the trailing end of one ribbon is soldered to the first contact of said second cell and the leading end of another ribbon is soldered to the second contact of said second cell. 13. Apparatus according to claim 12 wherein said soldering means is adapted to perform soldering by directing hot air at the ribbon(s) and cell in the soldering station. 14. Apparatus according to claim 12 further including means for raising the cell in the soldering station up out of contact with said conveyor means and holding it in raised position, for a finite period of time, and further including means for cooling the cell in the soldering station while it is in raised position. 15. Apparatus according to claim 13 wherein said conveyor means has holes therethrough, and further including means movable through said holes for raising a cell in the soldering station up off of said conveyor. 16. Apparatus according to claim 12 further including crimping means for crimping said ribbon immediately upstream of said cell in said"receiving station. 17. Apparatus according to claim 16 wherein said crimping means is operated substantially simultaneously with said cutting means. 18. Apparatus according to claim 12 wherein said conveyor means comprises individual nests each adapted to accommodate a single cell. 19. Apparatus according to claim 12 wherein said soldering means is movable along a path extending perpendicular to the plane of said conveyor means, and further including means for moving said soldering means close to and away from a cell that is located in said soldering station. 20. Apparatus according to claim 12 wherein said cutter means comprises cutter blade means movable along a path extending at a right angle to the plane of said conveyor means, whereby to move said cutting means into and out of engagement with said ribbon. 21. Apparatus according to claim 20 wherein said cutter means comprises top and bottom cutter members located above and below said conveyor and movable into overlapping relation with one another so as to sever a ribbon extending between them. 22. Apparatus according to claim 21 wherein said conveyor means comprises openings positioned to accept at least one of said top and bottom cutter members as said cutter members move into overlapping relation with one another. 23. Apparatus according to claim 20 further including crimping means for crimping said ribbon immediately upstream of said soldering station. 24. Apparatus according to claim 12 further including motorized means for rotating said roll of ribbon simultaneously with movement of said ribbon feeder means from said first position to said second position. 25. Apparatus for interconnecting cells comprising: (1) conveyor means for transporting cells along a first axis from a first cell-receiving station to a second cell-soldering station; (2) cell feeder means for feeding cells from a loading station to said conveyor means at said receiving station so that each cell fed by said feeder means rests on said conveyor means with its front or back contact engaging said conveyor means and the other of said front and back contacts facing away from said conveyor means; (3) drive means for moving said conveyor means stepwise on command so as to advance a cell carried thereby from said receiving station to said soldering station and then out from said soldering station to a collecting station located downstream of said receiving and soldering stations; (4) means for holding a supply roll of an elongate flexible electrical conductor in the form of a ribbon; (5) ribbon feeder means mounted for reciprocal movement parallel to said axis between a first position located upstream of said receiving station and a second position over the area occupied by a cell positioned at said soldering station, said ribbon feeder means including first clamp means for releasably holding the leading end of said ribbon as said feeder means moves from said first position to said second position and for releasing said ribbon so as to permit relative movement therebetween as said ribbon feeder means moves back to its said first position, whereby after said feeder means has returned to said first position from said second position, said ribbon will extend back from said soldering station through said receiving station; (6) second clamp means for releasably clamping the leading end of said ribbon to a cell positioned at said loading station; (7) means for cutting said ribbon immediately upstream of said receiving position; (8) soldering means for soldering the leading end of said ribbon to the upwardly-facing contact of the cell at the soldering station and also for simultaneously soldering the other contact of the same cell to the trailing end of another ribbon that underlies that cell; and (9) means for operating the foregoing means in sequence and repetitively so that the following steps occur: (a) the leading end of the ribbon from said roll is advanced so as to overlie a first cell in the soldering station and extend through the receiving station, (b) the leading end of the ribbon from said roll is held against movement relative to the cell in the soldering station, (c) the ribbon is cut so as to create a first cut-off section of ribbon that extends back across the cell in the soldering station through the receiving station, (d) the leading end of the cut-off section of ribbon is soldered to the upwardly facing contact of the cell in the soldering station, (e) a second cell is delivered to the receiving station so that it overlies the trailing end of the first cut-off section of ribbon at that station, (f) the conveyor is moved so as to transport the first cell out from the soldering station and move the second cell from the receiving station to the soldering station; and (g) the process is repeated starting with advancement of the new leading end of the ribbon so that it overlies the new cell in the soldering station, whereby the trailing end of one ribbon length will be soldered to the downwardly facing contact of said second cell and the leading end of another ribbon length will be soldered to the upwardly facing contact of said second cell. 26. Apparatus according to claim 25 for interconnecting solar cells that have substantially flat first and second opposite surfaces, first and second contacts bonded to said first and second surfaces respectively and a solder paste coating on selected aligned areas of said first and second contacts, further comprising first means for holding the trailing end of a first cut-off section of ribbon in contact with said selected areas of section of ribbon in contact with said selected areas of said first surface during soldering and second means for holding the leading end of a second cut-off section of ribbon in contact with said selected areas of said second surface during soldering. 27. Apparatus according to claim 26 wherein said first means comprises pedestal means movable into and out of engagement with said trailing end of said first cut-off section of ribbon. 28. Apparatus according to claim 26 wherein said second means form part of a hot air soldering mechanism. 29. A method for interconnecting a series of mutually adjacent solar cells by means of two flexible electrical conductors, with each cell having (1) a grid-type front contact comprising two bus bars intersecting a plurality of fingers, (2) a back contact, and (3) a solder material coated onto at least selected areas of each bus bar and selected areas of said back contact, with the selected areas of said back contact being aligned with the selected areas of said bus bars, characterized by the steps of: (1) advancing said two flexible conductors from supply rolls thereof and positioning said conductors in overlying relation to solder-coated areas of one of the front and back contacts of a first cell, (2) cutting said' conductors so as to form two separate conductor sections each having a leading end overlying said one contact of said first cell and a trailing end extending away from said first cell, (3) soldering said leading ends of said conductor sections to said one contact of said first cell, and (4) positioning a second cell over the trailing ends of said two conductor sections so that the solder coating on the other of the front and back contacts of said second cell are in engagement with said trailing ends, whereby when said trailing ends are subsequently soldered to said other contact of said second cell, said first and second cells will be connected electrically in series with one another. 30. Method according to claim 29 wherein said soldering is accomplished by directing hot air at said cell. 31. Method according to claim 29 wherein said conductors are held against selected areas by physical means as soldering is accomplished. 32. Method according to claim 31 wherein soldering is accomplished by directing hot air at said cell, and further wherein said physical means comprises means for directing hot air at one side of said cell. 33. Method according to claim 29 wherein said cells are disposed on a movable conveyor, and further wherein said cells are raised off of said conveyor during soldering step (c) . 34. Method according to claim 33 wherein said second cell is raised off of said conveyor during soldering step (c) by means that hold the trailing ends of said conductive ribbons against said second cell.

Inventors: BILLINGS RICHARD O, GONSIORAWSKI RONALD C, HARVEY DAVID S, HOLMES CARL KENNETH, BILLINGS, RICHARD, O., GONSIORAWSKI, RONALD, C., HARVEY, DAVID, S., HOLMES, CARL, KENNETH
Applicants: ANGEW SOLARENERGIE ASE GMBH, ASE AMERICAS INC, ANGEWANDTE SOLARENERGIE - ASE GMBH, ASE AMERICAS, INC.
Application year: 1994
Main classification: H01L31
IPC codes: H01L23, H01L31